Abstract
When and how planets form in protoplanetary disks is still a topic of discussion. Exoplanet detection surveys and protoplanetary disk surveys are now providing results that are leading to new insights. We collect the masses of confirmed exoplanets and compare their dependence on stellar mass with the same dependence for protoplanetary disk masses measured in ∼1–3 Myr old star-forming regions. We recalculated the disk masses using the new estimates of their distances derived fromGaiaDR2 parallaxes. We note that single and multiple exoplanetary systems form two different populations, probably pointing to a different formation mechanism for massive giant planets around very low-mass stars. While expecting that the mass in exoplanetary systems is much lower than the measured disk masses, we instead find that exoplanetary systems masses are comparable or higher than the most massive disks. This same result is found by converting the measured planet masses into heavy element content (core masses for the giant planets and full masses for the super-Earth systems) and by comparing this value with the disk dust masses. Unless disk dust masses are heavily underestimated, this is a big conundrum. An extremely efficient recycling of dust particles in the disk cannot solve this conundrum. This implies that either the cores of planets have formed very rapidly (<0.1–1 Myr) and a large amount of gas is expelled on the same timescales from the disk, or that disks are continuously replenished by fresh planet-forming material from the environment. These hypotheses can be tested by measuring disk masses in even younger targets and by better understanding if and how the disks are replenished by their surroundings.
Highlights
At least 30% of stars have planets (e.g., Zhu et al 2018) and, given current detection limits, it is plausible that planetary systems exist around every star
We collected the exoplanet and exoplanetary system masses and the masses of their host stars, and recalculated protoplanetary disk and disk-host star masses using the newly estimated distances based on Gaia parallaxes
We observe that single exoplanets around very low-mass stars can have masses almost as high as their host stars, whereas this is never observed in exoplanetary systems
Summary
At least 30% of stars have planets (e.g., Zhu et al 2018) and, given current detection limits, it is plausible that planetary systems exist around every star. Surveys of disks in several star-forming regions carried out with ALMA over the past few years, combined with optical to near-infrared spectroscopic surveys, are showing that the mass of the dust content of protoplanetary disks increases with stellar mass following a steeper than linear relation and decreases with time (e.g., Ansdell et al 2016, 2017; Pascucci et al 2016; Barenfeld et al 2016) These results show that disks around low-mass stars are less massive than the total mass in exoplanetary systems around low-mass stars, for example TRAPPIST-1 (Pascucci et al 2016; Testi et al 2016). We perform for the first time a detailed comparison of the disk masses measured with ALMA in young star-forming regions with the current information on planetary systems masses, as well as core masses, to infer information on planet formation timescales and processes
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